Question: Which of these numbers is prime? ${5,\ 25,\ 77,\ 84,\ 87}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 5 are 1 and 5. The factors of 25 are 1, 5, and 25. The factors of 77 are 1, 7, 11, and 77. The factors of 84 are 1, 2, 3, 4, 6, 7, 12, 14, 21, 28, 42, and 84. The factors of 87 are 1, 3, 29, and 87. Thus, 5 is a prime number.